Are we talking room thermostat? turn it down to 15 degrees and fire up your heating. If it is working your rads wont get hot as the thermostat will be telling the boiler/system that the room is already above 15 degrees and hence doesnt need heat.
If its not working (or the ambient temp is actually below 15 degrees!) the rads will get hot.
A point to bear in mind is here is no guarantee that the light is working. As they are for visual purposes only and have no effect on the working of the stat, because they require exrta wiring, installers don't always connect them up.
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